A column in a data.frame can be accessed using $, like so:

iris$Sepal.Length

#   [1] 5.1 4.9 4.7 4.6 5.0 5.4 4.6 5.0 4.4 4.9 5.4 4.8 4.8 4.3 5.8 5.7 5.4 5.1
#  [19] 5.7 5.1 5.4 5.1 4.6 5.1 4.8 5.0 5.0 5.2 5.2 4.7 4.8 5.4 5.2 5.5 4.9 5.0

The data.frame column can also be accessed with $ using backtick syntax:

`$`(iris, "Sepal.Length")

#   [1] 5.1 4.9 4.7 4.6 5.0 5.4 4.6 5.0 4.4 4.9 5.4 4.8 4.8 4.3 5.8 5.7 5.4 5.1
#  [19] 5.7 5.1 5.4 5.1 4.6 5.1 4.8 5.0 5.0 5.2 5.2 4.7 4.8 5.4 5.2 5.5 4.9 5.0

However, if a variable or function returning the string (in this case "Sepal.Length") is provided as the second argument, the results are different.

Examples

One might expect the same data.frame column to be returned when it is specified via a variable:

var = "Sepal.Length"
`$`(iris, var)
# NULL

or via a function:

`$`(iris, ifelse(TRUE, "Sepal.Length", NA))
Error in `$`(iris, ifelse(TRUE, "Sepal.Length", NA)) : 
  invalid subscript type 'language'

Question

Why does $ seemingly behave differently when its second argument is provided by a variable or a function, as opposed to a string?